Contamination of soil and ground water is presently one of the major environmental concerns in the United States and other industrial countries. Subsurface contamination has reached a level and extent at which it poses a serious threat to human health and the environment.
In recent years, a number of State and Federal Regulations have been developed to control and reduce subsurface contamination. Unfortunately, the technology available to clean-up the contamination sites is often not effective, and those that are effective, may be prohibitively expensive.
The ground water environment is generally divided into two major zones, (1) the unsaturated zone, also known as the "vadose zone" and (2) the saturated zone. The vadose zone extends from the ground surface down to the ground water table, while the saturated zone begins at the ground water table and extends to a further depth. The vadose zone may be further divided into additional subzones, but for purposes of the present invention, it will be considered as a single zone.
Since the vadose zone is the uppermost layer of the terrestrial environment, it contains the most important pathways for the toxic and hazardous chemicals to enter groundwater systems. As such, the removal of the toxic and hazardous chemicals in this zone is of paramount importance for all groundwater remediation.
The principal mechanisms that control the flow and transport of chemicals in the vadose soil zone are mass flow, liquid diffusion, and vapor diffusion. Further, treatment of the vadose zone is high priority in any ground water remediation action since transport of the contaminants from the unsaturated to the saturated zones occurs continuously by percolation and vapor transport. Studies have shown that it is less costly to remove volatile organic compounds (VOC) from the vadose zone than to pump and treat contaminated ground water. For this reason, technology is being developed for the in situ removal of VOC from the vadose zone. Such treatment technologies include vapor extraction, biodegradation, soil washing and thermal treatment.
Several factors influence volatilization from soil. Temperature is one of the major factors that must be taken into account. Volatilization increases significantly as temperature increases because of the increase in vapor density and thereby the increase of vapor diffusion. Chemical concentration increases will increase volatilization considering that the chemical's vapor density is not saturated. Decreasing water content increases the vapor diffusion and volatilization. Finally, the wind speed increases volatilization because it improves mixing with the atmosphere and can increase volatilization.
Specifically, vapor extraction is a process for the in situ removal of volatile organic compounds (VOC) by mechanically extracting soil gas from the vadose zone. Specifically, one or more vertically oriented perforated vent wells are installed in the contaminated zone in the ground, and air is forced to travel through the pore space in the soil, causing volatilization of the liquid and adsorbed volatile organic compounds. The extracted soil gas is then either vented to the atmosphere or into an emission control system, depending on the concentration.
Two major variations of the vapor extraction process have been demonstrated successfully, namely an in situ air stripping process and a vacuum extraction process.
In the in situ air stripping process, a series of interconnected air injector vents are supplied with forced air by an above ground blower and manifold system that forces the air into the soil through the perforated vent wells. A separate blower and manifold system is used to apply negative pressure to air extraction vents to withdraw the soil gas. The injection and extraction vents are located alternately within the array of vent wells on the site. To achieve a degree of flow containment, extraction vents are placed on the perimeter of the area being treated.
Although various tests have found the air stripping process to be effective in removal of VOC from the vadose zone, these tests have identified an important limitation to its use. Specifically, the system functions best with high permeable soil such as the loose, sandy soils present. It was therefore concluded by the testers that the process would be much less effective in tightly packed soils and in soils with a high clay content.
The second variation of the vapor extraction process is vacuum extraction. As with in situ air stripping, vacuum extraction provides at least one perforated vent well installed in the vadose soil zone. A vacuum pump is installed on the wells and induces a negative pressure gradient around the well to remove the VOC along with the soil gas. Various examples of usage thereof include leakage of carbon tetrachloride from an industrial tank farm into a clay-like residual soil in a karst area in Puerto Rico, and surface spillage of acetone and methylene chloride around underground storage tanks, in which a vacuum extraction system was used to reduce the contamination to an acceptable limit. Even though the soil at the sites consisted of mixed silts, sands and clays, the vacuum wells developed a radius of influence of up to 17 feet. High recovery rates have also been reported using vacuum extraction to withdraw gasoline and residual hydrocarbons from gravel backfill around leaking underground storage tanks.
A summary of the design considerations of vacuum extraction systems is provided in the article "Vacuum Extraction From Soil" by M. B. Bennedsen, Pollution Engineering, Feb. 1987, pages 66-68. In the article, it is concluded that vacuum extraction has been used successfully on soils in a range of permeability from 1.times.10.sup.-4 to 1.times.10.sup.-8 cm/sec. Other important design parameters include the depth to the water table, the air emission control and short circuiting of air from the ground surface. In order to control the latter, a cover is sometimes installed over the area.
As discussed above, biodegradation is another process which has effectively been used in the treatment of soils contaminated with hazardous organic compounds. Specifically, with biodegradation or bioremediation, the environmental conditions in the soil are altered to enhance microbial catabolism or to cometabolise the organic contaminant, thus transforming it into a simpler, non-toxic product. In most applications, indigenous microorganisms are utilized, although seeding of the soil with indigenous microorganisms has also been used where natural occurring organisms are unable to degrade the contaminants.
Microorganisms can be classified into three main categories, namely (1) aerobic, which grow only in the presence of oxygen, (2) anaerobic, which grow only in the absence of oxygen, and (3) facultative anaerobic, which can grow either in the absence or presence of oxygen. The biodegradation method which has been found most effective in treatment of the vadose soil zone has been the aerobic microbial process. With this process, oxygen and often nutrients are injected or infiltrated into the subsurface environment, using wells or a percolation process. For example, wells are drilled into the soil and nutrients for feeding the microbes are dropped down into the well, or microbes are seeded in the well. Thereafter, the microbes are blown outwardly by forced air or the like. A concise summary of the major factors which affect the rate of biodegradation in the vadose zone are described by R. L. Valentine et al in "Biotransformation" in Vadose Zone Modellino of Organic Pollutants, edited by Stephen Hern et al, Lewis Publishers, Inc. Mich., Chapter 9 (1986), and include: (1) pH, (2) temperature, (3) water content, (4) carbon content, (5) clay content, (6) oxygen, (7) nutrients, (8) the nature of the microbial population, (9) acclamation and (10) concentration.
A number of investigators have reported successful application of the in situ biodegradation process to treatment contaminated soils and have concluded that it is often cost effective and reduces site disruption. Some important limitations have also been identified, such as reaction kinetics, low substrate concentration and slow degradability of certain compounds.
R. Wetzel et al in "Demonstration of In Situ Biological Degradation of Contaminated Ground Water and Soils", Sixth National Conference on Management of Uncontrolled Hazardous Waste Sites, Washington, D.C. (1985) describe a demonstration at Kelly Air Force Base, Texas, to treat contaminants consisting of hydrocarbons, aromatics and halogenated organics. A major limiting factor of the remediation is cited as the low permeability of the fine-grained soil layers present at the site.
Further, V. Jhaveri et al describe in "Bioreclammation of Ground and Ground Water by In Situ Biodegradation" Case History, Sixth National Conference on Management of Uncontrolled Hazardous Waste Sites, Washington, D.C. (1985) report the bioreclammation of a site in New Jersey contaminated with methylene chloride, n-butyl alcohol, acetone and dimethylaniline. After three years of in situ aerobic biological treatment, the contaminant plume was reduced by 90%.
Still further, P. Yaniga et al in "Aquifer Restoration Via Accelerated In Situ Biodegradation of Organic Contaminants", Seventh National Conference on Management of Uncontrolled Hazardous Waste Sites, Washington, D.C. (1986), in describing the reclamation of an aquifer contaminated with benzene, toluene, and xylene using biodegradation, emphasize the importance of oxygenating the subsurface environment. It is reported therein that superior rates of biodegradation using hydrogen peroxide as an oxygen donor result in comparison with a more traditional technique of air sparging.
In the method of thermal treatment or thermal soil decontamination, a contaminated soil is heated to a temperature of 400.degree. C. to 700.degree. C., which is sufficient to evaporate or to pyrolize the organic contaminants. The gaseous products are then removed by convection. After dust removal from the gas stream, the organic products are completely destroyed in an after burner at temperatures as high as 1200.degree. C. Consequently, the residual soil is ready for reuse after reconditioning, such as remoisturing. However, this method is in general unsuited for the removal of inorganic contaminants because of low volatility. Further, some inorganic compounds such as complexed cyanides and the more volatile metals such as mercury will decompose to gaseous products or evaporate partially under these conditions.
Thermal soil decontamination methods may be divided into two categories, namely rotary kiln fired decontamination and in situ decontamination. In the rotary kiln method, the required heat is transferred by direct or indirect firing of the soil. The fluidized bed technique is generally employed in direct heating, whereas infrared radiation on a conveyor belt or using hot oil in an extruder type reactor has been used for indirect heating. The in situ method applies radio frequency techniques. A system of electrodes is implanted into the ground to heat the soil to a predetermined temperature to vaporize the contaminants which are then vented by convection.
Another thermal treatment system is the thermal desorption-UV photolysis process described by S. G. DeCicco in "Transportable Hybrid Thermal Treatment System", 24th AIChE/ASME National Heat Transfer Conference, Pittsburgh, Pa., pages 407-412 (1987).
Although the approach seems to be promising in soil decontamination, the difficulty lies in the basic understanding of transport phenomena and chemical processes associated with evaporization/pyrolysis of organic matter such as organochlorine compounds. However, more research is needed to model the thermal desorption of contaminants from soils under a wide variety of thermal conditions.
In general, the in situ approach is less expensive than the rotary kiln method, but the latter has proved more effective since the kiln agitation reduces the amount of contaminant trapped in the soil pores.
Paramount among the limitations of the above existing and emerging treatment technologies in the vadose zone is the permeability of the soil formation being treated. The efficiency of the aforementioned in situ treatment processes all decrease as the soil permeability decreases. For soils with low permeabilities the existing processes are largely ineffective. Low soil permeability may be caused by a number of factors, including high clay content, high soil density and high fluid viscosity. Therefore, the effectiveness of virtually all in situ treatment processes in the vadose zone can be enhanced by increasing the permeability of the soil formation.
Unrelated to the removal of hazardous and industrial waste, it is also known to provide hydraulic fracturing in the petroleum industry to enhance well production. See, for example, the book Hydraulic Fracturing by G. C. Howard et al, Millet the Printer, Dallas, Tex., 1970. This is also related to pressure grouting procedures used to increase soil strength and decrease permeability around various engineering works, such as dam foundations and tunnels, which also uses water or other liquid agents.
In hydraulic fracturing, a section of an oil well is sealed off with pressure packers, and water is then injected until the pressure is sufficient to initiate fracture of the surrounding rocks. Once a crack is formed in the rocks, it will continue to propagate as long as the water pressure is greater than the stress normal to the plane of fracture. However, this process has only been used for fracturing rocks.
The theory describing hydraulic fracturing was first set out by M. King Hubbert in "Mechanics of Hydraulic Fracturing" in Trans. Am. Inst. Min. Engrs., Volume 210, pages 153-168 (1957) in which various modes of failure are discussed. Advances have also been made by subsequent investigators, such as R. O. Kehle in "The Determination Of Tectonic Stresses Through Analysis of Hydraulic Well Fracturing", J. Geophys. Res., Volume 69, pages 259-273 (1964) and P. L. Bertrand in "Note Seur L'Equilibre Elastigue D'Un Milieu Indefini Perce D'Une Cavite Cylindrique Sous Pression", Annls. Ponts. Chauss., Volume 134, pages 473-522 (1964), who employ a stress concentration concept to analyze fracture conditions.